Plasmonic Semiconductor Nanoparticles in a Metal–Organic

Article pubs.acs.org/cm

Plasmonic Semiconductor Nanoparticles in a Metal−Organic Framework Structure and Their in Situ Cation Exchange Andreas Wolf, Lisa Diestel, Franziska Lübkemann, Torben Kodanek, Tarek Mohamed, Jürgen Caro, and Dirk Dorfs* Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover, Callinstrasse 3A, 30167 Hannover, Germany S Supporting Information *

ABSTRACT: Composites of nanoparticles (NPs) in metal− organic frameworks (MOFs) combine the versatile properties of NPs with the defined porosity of MOFs. Here, we show the encapsulation of plasmonic semiconductor NPs in ZIF-8 crystals. Both p-type and n-type doped plasmonic semiconductor NPs (consisting of Cu2−xSe and indium tin oxide, respectively) are encapsulated. The plasmonic and structural properties of each system are preserved during the formation of the composites. Furthermore, we demonstrate the accessibility of the NPs integrated in ZIF-8 via the successful first-time in situ cation exchange of MOFembedded Cu2−xSe NPs to HgSe NPs and Ag2Se NPs. This ion exchange occurs without influencing the composition or structural integrity of the metal−organic framework. This approach hence allows a fixation of plasmonic nanoparticles avoiding strong plasmon−plasmon coupling but still keeping the plasmonic nanoparticles accessible.

1. INTRODUCTION The discovery of the origin of the near-infrared (NIR) absorption band in copper chalcogenides has sparked intensive research in degenerately self-doped semiconductor nanoparticles that exhibit localized surface plasmon resonances (LSPRs).1−9 Compared to classical metal nanoparticles (NPs), these materials opened a further path for tuning the LSPR. While the tuning for metal NPs is limited to size, shape, and material choice during synthesis, the LSPR of self-doped copper chalcogenides can further be tuned by postsynthetic oxidation and reduction.2 In addition, the cation exchange with different cations allows the shift of the LSPR maximum.10 The accessibility of the copper chalcogenides encouraged studies that use these particles as a starting or intermediate structure for various cation exchange experiments. These reactions can lead to crystallographic phases and shapes that are not directly accessible.7,11−20 The LSPR in Cu2−xSe is caused by p-type selfdoped free charge carriers.2 In contrast to that, indium tin oxide (ITO) is n-type doped. The doping of indium oxide with Sn4+ causes a material composition-dependent charge carrier density and thus a tunable NIR LSPR that is stable under ambient conditions.21 Hence, these two systems are excellent examples of plasmonic semiconductors with a LSPR in the NIR that are covering p-type as well as n-type doped semiconductors. Growing shells on a desired core nanoparticle is common practice in nanotechnology for enhancing the properties of the nanoparticle or protecting the core.22,23 Our group previously showed the synthesis of plasmonic Cu2−xSe@ZnS NPs. The ZnS shell allows the Cu2Se core to be oxidized to Cu2−xSe and exhibit a strong plasmon absorption band in the NIR after the © 2016 American Chemical Society

oxidation. However, at the same time, the shell protects the oxidized core from reduction with several reducing agents.24 Nanoporous metal−organic framework (MOF) structures have been studied and applied in various applications over the past decade.25,26 Their homogeneous structure with a defined cavity and gate size fueled the research efforts in this area. Combining the properties of MOFs with the unique properties of nanosized materials was a logical consequence of achieving highly functional composite materials.27−34 Reports on the formation of such composites can mostly be grouped into two approaches. On the one hand, the infiltration of a ready-made MOF structure with the precursor of the desired material, and the subsequent precipitation or reduction inside the existing pores. On the other hand, the growth of a porous MOF structure around the ready-made NPs, whose properties are pretailored, is allowing the NPs not to occupy the pores but rather to be enwrapped by the MOF.34 While the former approach is limited by pore size, the later allows composite structures, which host NPs with various dimensions and tuned properties as needed for the desired application. A further functionalization step was taken by Zhang et al., who achieved, starting from metal-NP@MOF structures, mesoporous MOFs by removal of the metal NPs via etching.35 We show here, to the best of our knowledge, the first-ever incorporation of plasmonic semiconductor NPs into a MOF structure. While previous works have shown the incorporation Received: August 17, 2016 Revised: September 30, 2016 Published: September 30, 2016 7511

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials

particles were cleaned by being washed three times with fresh MeOH. For cation exchange with Ag+, a 25 mM AgNO3 solution in MeOH was used and the sample was further treated as described above. 2.4. UV/Vis/NIR Absorbance Spectra. Samples were diluted with the chosen solvent and placed in a quartz glass cuvette (4 and 1 mm path lengths). The spectra were recorded in standard transmission mode or the center mounting position of an Agilent DRA-2500 Ulbricht sphere, which was mounted in an Agilent Cary 5000 UV/vis/ NIR spectrophotometer. 2.5. Electron Microscopy. Transmission electron microscopy (TEM) measurements were performed on an FEI Tecnai G2 F20 instrument, equipped with a field emission gun operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDX) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements were performed with a JEOL JEM2100F instrument, operated at 200 kV and equipped with a field emission gun. EDX copper mapping cannot be conducted reliably with this setup, because of the high level of background noise caused by copper pole pieces of the transmission electron microscope. All TEM and EDX sample preparation was conducted by dropping thrice 10 μL of the composite particle solution on a carbon-coated copper grid that is placed on filter paper (gold grid for EDX measurements); no sample thinning was performed. Scanning electron microscopy (SEM) measurements were performed on a JEOL-JSM-6700F scanning electron microscope, operated at 2.0 kV. Samples were prepared by drop casting a diluted solution on a graphite sample holder. 2.6. Powder X-ray Diffraction Analysis. A Bruker D8 Advance instrument in reflection mode was used for XRD pattern acquisition. It was operated at 40 kV and 40 mA, with Cu K-alpha radiation. Samples were prepared by drying the NP solution on a single crystalline sample holder. 2.7. Atomic Absorption Spectroscopy. The copper concentration was determined by atomic absorption spectroscopy (AAS) by applying a Varian AA 140 spectrometer. The solvent of the sample was evaporated, and the precipitate was dissolved in aqua regia (1 part HNO3 and 3 parts HCl) and diluted with deionized water. The measurements were taken using an acetylene flame at 324.8 nm.

of several plasmonic metal nanoparticles, plasmonic semiconductor NPs have not been successfully built into MOF NPs to date. In this work, the formation of composite particles consisting of multiple Cu2−xSe NPs or ITO in one ZIF-8 particle resulting in Cu2−xSe@ZIF-8 or ITO@ZIF-8 composites, respectively, will be shown. In the ZIF-8 lattice, the Zn2+ ions are interconnected by methylimidazolate anions forming a sodalite (SOD) topology.36 The resulting composite particles exhibit a strong absorption band in the NIR originating from the LSPR of the incorporated NPs. Oxidation permits the tuning of the Cu2−xSe LSPR inside the Cu2−xSe@ZIF-8. While postsynthesis cation exchange reactions of the metal nodes of MOFs have been reported previously,37−39 no reports are known for the selective cation exchange of NPs embedded in MOFs. We here report for the first time a cation exchange of NPs inside a NP@MOF composite. Specifically, we exchange the Cu2−xSe NPs inside a Cu2−xSe@ZIF-8 composite with Ag+ and Hg2+ ions, resulting in HgSe@ZIF-8 and Ag2Se@ZIF-8 composite particles, without modifying the ZIF-8 host structure. This further demonstrates the accessibility of the plasmonic NPs, while the fixation within the MOF framework prevents an agglomeration of them.

2. EXPERIMENTAL SECTION Additional information about used materials, the synthesis of quasispherical Cu2−xSe NPs, and ITO NPs can be found in the Supporting Information. 2.1. Phase Transfer of Cu2−xSe and ITO NPs. We adapted a PVP phase transfer approach developed for Fe3O4 NPs.28 A 0.4 mL Cu2−xSe NP solution in toluene (corresponds to 3.5 mg of Cu2−xSe NPs as determined by AAS) was precipitated by addition of MeOH (180 μL) and EtOH (200 μL) and centrifugation. The precipitate was redissolved in 5 mL of CHCl3. While the mixture was being stirred, PVP 55k (63 mg) dissolved in CHCl3 (2.5 mL) was added to the Cu2−xSe particles. The mixture was left to stir for 10 days. To remove excess PVP, the NPs were precipitated with n-hexane (400 μL of nhexane/mL of NP/PVP solution) and centrifugation (14500g). After redissolution in CHCl3 (3 mL), the precipitation was repeated with 750 μL of n-hexane and the NPs were finally redispersed in MeOH (3 mL). The ITO NPs were collected from a 150 μL solution in toluene by centrifugation (5000g for 5 min), after precipitation with EtOH (100 μL). The precipitate was redissolved in CHCl3 (150 μL) and precipitated again with 500 μL of EtOH (centrifugation at 10000g for 10 min). The particles were then transferred inside the glovebox, redissolved in CHCl3 (5 mL), and treated under a nitrogen atmosphere henceforward. The ITO NPs were cleaned by precipitation with n-hexane (3.5 mL) and centrifugation (15 min at 14500g). After that, they were redissolved in CHCl3 (2.8 mL), again precipitated with n-hexane (0.7 mL), and centrifuged (15 min at 14500g). The PVP-coated ITO NPs are finally dissolved in MeOH (2.4 mL). 2.2. Growth of Cu2−xSe@ZIF-8 and ITO@ZIF-8 Composite Particles. The encapsulation of plasmonic Cu2−xSe and ITO NPs into ZIF crystals was adapted from a method previously reported for different NPs;28 3 mL of PVP-coated NPs in MeOH (concentration of 0.9 mL of NP solution and 2.1 mL of MeOH) was mixed with 15 mL of a 30 mM HMIM in MeOH solution and ultrasonicated for 5 min. Fifteen milliliters of a 30 mM Zn(NO3)2 in MeOH solution was added in one step. After being shaken for 3 s, the reaction mixture was left for 24 h without being agitated. The ready composite particles were cleaned by repeated centrifugation (1000g) and redispersed in MeOH (6 mL). 2.3. Cu2−xSe Cation Exchange with Ag+ and Hg2+ inside the ZIF-8 Composite. For cation exchange with Hg2+, 40 μL of a 25 mM HgCl2 solution in MeOH was added to 500 μL of a methanolic solution of the Cu2−xSe@ZIF-8 composite solution. After 90 min, the

3. RESULTS AND DISCUSSION 3.1. Cu2−xSe@ZIF-8 Composite Particles. Metal−organic framework ZIF-8 was grown around quasi-spherical Cu2−xSe NPs while their plasmonic properties were preserved. Therefore, the Cu2−xSe NPs were synthesized by adapting a strategy previously reported by Deka et al.40 The average size of the pristine, quasi-spherical, and OLAcapped Cu2−xSe NPs of 13.1 ± 1.5 nm is determined by TEM (see Figure S1). These particles are coated with polyvinylpyrrolidone (PVP) 55k by being stirred in a PVP CHCl3 solution for several days to ensure successful functionalization with this amphiphilic polymer. The subsequent removal of excess amounts of PVP by repeated washing is essential. Previous works on NP@ZIF-8 composite materials have shown that excess amounts of free PVP hinder the integration of NPs into the ZIF-8 particles, due to competitive adsorption of free PVP molecules on the forming ZIF-8 crystal.28 For a typical synthesis, a Cu2−xSe@PVP NP dispersion in methanol (MeOH) is mixed with a 30 mM 2-methylimidazole (HMIM) solution in MeOH, and subsequently, a 30 mM ZnNO3 solution in MeOH is quickly added. After a short mixing, the reaction solution is left without agitation to allow the undisturbed growth of the composite particles. After 2−3 min, the reaction solution becomes continuously hazy. After 24 h, a brown-greenish-colored precipitate can be observed at the bottom of the reaction vial, while the supernatant is clear and colorless. 7512

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials

Figure 1. (a−c) TEM and (d−f) SEM micrographs of (a and d) typical Cu2−xSe@ZIF-8 particles, (b and e) Cu2−xSe@ZIF-8 particles with a low Cu2−xSe NP concentration, and (c and f) Cu2−xSe@ZIF-8 composite particles resulting from 50 mM HMIM and ZnNO3 precursor solutions. The insets show the magnifications of single composite particles.

NPs that are well dispersed inside the ZIF-8 crystal, as also seen in the TEM bright field images (Figure 1). The larger ZIF-8 crystals, however, exhibit only low contrast, because of their comparably low electron density. As expected, zinc (Zn) can be detected in the whole ZIF-8-based composite particle. The Se mapping of the composite particle shows that selenium (Se) can be detected only in the corresponding high-contrast areas. Thus, we can confirm that the observed NPs are indeed the introduced Cu2−xSe NPs. In the TEM bright field micrographs (Figure 1a) as well as in the HAADF-STEM micrographs (Figure 2), one can see that the embedded Cu2−xSe NPs are well-dispersed inside the ZIF-8 crystals. The TEM and SEM micrographs in panels b and e, respectively, of Figure 1 show the resulting composite particles when only half of the concentration of Cu2−xSe NPs is used in the ZIF-8 synthesis batch, while all other synthesis parameters are kept the same. The composite particles have an average size of 286 ± 24 nm (smallest distance from edge to edge), hence showing that the size of the composite particles is independent of the concentration of the introduced Cu2−xSe NPs. The same applies for the shape, as also mainly rhombic dodecahedron particles and some cubic-shaped particles are formed. However, it can be observed that the embedded Cu2−xSe NPs are located in a smaller radius around the center of the composite particle. This concentration of the embedded NPs results in a larger NPfree ZIF-8 shell, showing the expected faster NP depletion during ZIF-8 growth. To create larger composite particles, the concentrations of the ZIF-8 precursors (HMIM and ZnNO3) are increased to 50 mM while all other reaction conditions are kept constant. The resulting Cu2−xSe@ZIF-8 particles are nearly double in size (619 ± 55 nm, smallest distance from edge to edge), as can be seen in the corresponding TEM and SEM micrographs (panels c and e, respectively, of Figure 1). The particles are mostly rhombic dodecahedral and cubic in shape. It is furthermore evident that the radii in which Cu2−xSe NPs are observed are larger, and no NP-free ZIF-8 particles can

Transmission electron microscopy reveals the formation of 281 ± 23 nm (smallest distance from edge to edge) sized Cu2−xSe@ZIF-8 composite crystals, with multiple smaller, higher-contrast Cu2−xSe NPs embedded (Figure 1a). Using in addition the associated SEM micrograph (Figure 1d), we can see that the majority of the ZIF-8-based composite particles exhibit a rhombic dodecahedron shape. However, other shapes such as cubes are also present as described by Cravillon et al.41 The composite particles agglomerate easily during dispersion. This follows from the TEM and SEM micrographs showing mostly agglomerated particles, but it can also be deduced from the complete self-precipitation of the particle dispersions within 1−2 h. Some particles further exhibit crystal twinning. Because the supernatant of the synthesis, after reaction for 24 h, is colorless and nearly no free Cu2−xSe NPs can be detected in the TEM micrographs, we conclude that with the described synthesis parameters a quantitative integration of the Cu2−xSe NPs into the ZIF-8 crystals can be achieved. The TEM micrographs further show that multiple Cu2−xSe NPs are always integrated in a single ZIF-8 crystal and that these are not agglomerated as described in several previous reports.42,43 Figure 2 shows the elemental mapping of a Cu2−xSe@ZIF-8 composite particle, established by EDX in the scanning mode of the TEM (STEM). The HAADF-STEM micrograph (Figure 2) of the analyzed composite particle shows the brighter Cu2−xSe

Figure 2. HAADF-STEM micrograph of a Cu2−xSe@ZIF-8 composite particle and the corresponding EDX mappings for Zn and Se. 7513

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials be found. Pure ZIF-8 particles that are received with the described synthesis method have a size of 604 ± 90 nm (see TEM micrographs in Figure S2). The large size of the pure ZIF-8 particles compared to that of the composite particles shows that the addition of the Cu2−xSe NP solution affects the crystallization behavior of the ZIF-8 hosting crystal. Previous works in other NP@MOF composites showed, however, that the crystallization mechanism is homogeneous nucleation and that the NPs are continuously adsorbed on the growing crystal surface until their depletion, rather than acting as centers for heterogeneous nucleation.28 We additionally observed that reducing the NP concentration in the ZIF-8 synthesis batch does not change the particle size of the resulting composite. We speculate, therefore, that small amounts of copper ions in solution, set free due to the partial oxidation of the Cu2−xSe NPs, promote ZIF-8 nucleation and result in composite particles that are smaller than those of pure ZIF-8. Nevertheless, with the method presented herein, the overall composite particle size as well as the NP loading can be adjusted in a controlled way by simply adapting the NP concentration as well as the ZIF-8 precursor concentration. The LSPR in the Cu2−xSe NPs is caused by a copper deficiency in the system that can, for example, be induced by postsynthetic oxidation of the stoichiometric Cu2Se system. Recent XPS and Raman spectroscopy studies by the Manna group have shown that only Cu(I), not Cu(II), is present in the crystal and that during oxidation the selenide ions are partially forming diselenides.4,13,44 Exposure to oxygen or other oxidizing agents leads to self-doping that causes an increased density of free p-type charge carriers, resulting in the development of the LSPR.2 This can be recognized as an increasing intensity of the absorption band in the NIR, which hypsochromically shifts with an increasing copper deficiency. Typically, the absorption band reaches a maximum around 1000−1200 nm after oxidation for several days under air.2,10 The pristine Cu2−xSe NPs show only a very weak LSPR at relatively long wavelengths. When they are exposed to air, however, the absorption band evolves and reaches a maximum at 1190 nm after 2 days (see Figure S3). The PVPfunctionalized NPs show a weak LSPR band with a maximum at roughly 1700 nm before their exposure to air (see Figure S3). When exposed to air, these NPs develop a stronger LSPR with a maximum comparable to that of the pristine NPs in toluene. The Cu2−xSe@ZIF-8 composite particles strongly scatter because of their size but also exhibit an absorbance maximum at 1450 nm in normal transmission mode (see Figure 3). Measuring only the absorption of the same sample, by employing an integrating sphere to account for the scattering, results in a spectrum that correspond to that of typical Cu2−xSe NPs. This further shows that while the NPs are fixed in the MOF framework no agglomeration of the NPs occurred, as this would have caused a strong bathochromic shift and broadening of the resonance band. When the NPs are exposed to air, a hypsochromic shift for the LSPR of the composite particle can be detected, as seen for the pristine and PVP-functionalized NPs, and as previously reported.2,10 Following this finding, it can be stated that the ZIF-8 crystal around the plasmonic Cu2−xSe NPs does not significantly change their optical properties. However, the oxidation of the Cu2−xSe NPs is slowed, which is most likely due to the kinetic obstruction due the pore system of the surrounding ZIF-8 host crystal. A further reason for the weak bathochromic shift in comparison to the pristine and PVP-coated NPs could also be a change in the

Figure 3. (a) Absorbance spectra of Cu2−xSe@ZIF-8 composite particles after different periods of exposure to air, measured in transmission mode (solid lines) and absorption spectra in the integrating sphere (dotted lines). (b) XRD patterns of pristine Cu2−xSe and ITO NPs, ZIF-8 particles, and Cu2−xSe@ZIF-8, ITO@ ZIF-8, HgSe@ZIF-8, and Ag2Se@ZIF-8 composite particles (magnification of the lower-intensity range for pure ZIF-8 and composites, to highlight the reflexes of the embedded NPs; the full intensity range is depicted in Figure S4) . The reference structural data show the simulated ZIF-8 pattern, fcc Cu2−xSe berzelianite (PDF Card No. 01072-7490), bcc (In1.88Sn0.12)O3 (PDF Card No. 01-089-4598), fcc HgSe tiemannite (PDF Card No. 00-008-0469), and orthorhombic Ag2Se naumannite (PDF Card No. 01-089-2591).

dielectric environment of the NPs due to the surrounding ZIF8 crystal. The refractive index for dense ZIF-8 films given in the literature ranges from 1.5445 to 1.584.46 Because of the similarity to the refractive index of toluene (1.4950)47 and methanol (1.3289),47 the effect would be relatively small in comparison to the changes that even small amounts of copper deficiencies cause in the maximal position of the LSPR. Furthermore, the UV/vis/NIR absorbance measurements show no signs of composite decomposition when NPs are exposed to lamp light or daylight. The X-ray powder diffraction (XRD) pattern (see Figure 3) of the pure Cu2−xSe NPs shows, as previously reported, the cubic berzelianite structure.40 Comparing the XRD patterns (see Figure 3 for the lower quarter of the intensity region and Figure S4 for the full intensity range) of pure ZIF-8 particles and of Cu2−xSe@ZIF-8 particles, one can state that the intensity ratios of the diffraction peaks, which are assigned to ZIF-8, are slightly changed for the composite particles. Additionally, at a 2Θ of 26.9°, a slight broadening of the 7514

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials diffraction peak can be seen, and weak additional diffraction peaks at a 2Θ of 44.6° can be observed for the composite. These correspond to the diffraction peaks that are associated with the berzelianite structure of the Cu2−xSe NPs, thus confirming that the integrated NPs did not undergo a phase change during the composite synthesis. The large difference in the intensity ratios between the ZIF-8- and Cu2−xSe-assigned diffraction peaks is caused by the small volume fraction and crystal size of Cu2−xSe compared to those of ZIF-8 in the Cu2−xSe@ZIF-8 composite particles. In this paper, we describe for the first time the successful synthesis of plasmonic Cu2−xSe@ZIF-8 composite particles. The characterization showed that the embedded Cu2−xSe NPs are integrated without agglomeration and retained their structural properties and LSPR. Because of the universality of the approach, which applies PVP to integrate NPs,28 it should also be possible to achieve further copper chalcogenide@ZIF-8 composites, e.g., with Cu2−xS, Cu2−xTe, or ternary copper chalcogenides like CuInS2 for applications in photocatalysis, solar energy conversion, etc.7,48 3.2. ITO@ZIF-8 Composite Particles. For the synthesis of ITO@ZIF-8 composite particles, we synthesized first ITO NPs, doped with 10% Sn following a method from Kanehara et al.21 The ITO NPs stabilized with OLA were coated with PVP and transferred to MeOH, which is similar to the described method for the Cu2−xSe NPs. The ITO NPs have an average size of 8.7 ± 1.5 nm (see the TEM micrograph in Figure S5). The ITO@ZIF-8 composites were synthesized as described for the Cu2−xSe@ZIF-8 particles. The resulting precipitate of the composite particles looks white in contrast to the Cu2−xSe@ZIF-8 composites, because of the lack of ITO absorption features of ITO in the visible range of the spectrum. The ITO@ZIF-8 composite particles are larger than the comparable Cu2−xSe@ZIF-8 particles, as one can see in the TEM and SEM micrographs in panels b and c, respectively, of Figure 4. The TEM micrographs further show the good dispersion of the ITO NPs throughout the whole ZIF-8 host crystal. The higher dispersion is likely due to the slightly smaller NP size (compared to Cu2−xSe), but possibly also influenced by a very good colloidal stabilization of the ITO NPs with PVP. The elemental mapping (Figure 4d) indicates the expected assignment of the higher-contrast particles to the ITO NPs containing indium (In) and tin (Sn). XRD analysis (see Figure 3 and Figure S4) shows, furthermore, that these particles did not undergo changes in their body-centered crystallographic structure throughout the phase transfer and composite formation. The optical measurements of the pristine NPs in toluene show a strong LSPR with a maximum at 1880 nm, which originates from the heavy n-type doped free charge carrier concentration.21 The coating with PVP and the transfer to MeOH result in a small hypsochromic shift to 1840 nm that is likely caused by the smaller refractive index of MeOH in comparison to that of toluene. The absorbance spectra of the ITO@ZIF-8 composite particles show a strong scattering, caused by the large particles, in the NIR and the visible spectral range as well as a clear maximum at 1856 nm. The absorption spectrum is comparable to that of the pristine and PVP-coated NPs and shows no sign of agglomeration. The LSPR maximum, however, is bathochromically shifted to 1856 nm in comparison to that of the PVP-coated particles. This is expected because the refractive index of ZIF-8 is higher than that of MeOH. Similar to the Cu2−xSe@ZIF-8 composites, no decomposition can be observed under lamp or daylight exposure.

Figure 4. (a) Absorbance spectra of the as-synthesized ITO NPs in toluene (black), the ITO-PVP NPs in MeOH (red), and the ITO@ ZIF-8 composite particles (blue) as well as the absorption spectra (measured in an integrating sphere) of the ITO@ZIF-8 composite particles (dotted line). (b) TEM and (c) SEM micrographs of ITO@ ZIF-8 particles. (d) HAADF-STEM micrograph and corresponding elemental mappings for Zn, In, and Sn of a typical ITO@ZIF-8 composite particle.

We demonstrate the integration of plasmonic ITO NPs into ZIF-8 crystals while preserving their plasmonic properties. In contrast to the Cu2−xSe@ZIF-8 particles, the embedded ITO NPs show a fixed LSPR position that cannot be changed by oxidation. Therefore, small variations caused by the changes in the dielectric environment can be detected with greater certainty, in comparison to that for the Cu2−xSe@ZIF-8 composite. This fact opens the application for size exclusive LSPR-based optical sensing in the NIR region that is not accessible for metal NP LSPR sensing. Furthermore, this 7515

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials

Figure 5. HAADF-STEM micrograph and EDX mapping of Zn, Se, Hg, or Ag for the (a) HgSe@ZIF-8 and (b) Ag2Se@ZIF-8 composite particles obtained by ion exchange.

finding shows that it is likely that other n-type doped plasmonic oxide particles6,8 can also be integrated into ZIF-8 with this universal approach. 3.3. In Situ Cation Exchange of NPs inside the Cu2−xSe@ZIF-8 Composite. Copper chalcogenide systems have been shown to be a very versatile starting material (or intermediate) for cation exchange reactions.7,49−51 Consequently, we analyzed the behavior of the Cu2−xSe@ZIF-8 composite particles upon their exposure to Ag+ and Hg2+ ions in the solvent methanol. As demonstrated in Figure S6, the addition of both ions leads to the complete disappearance of the LSPR, as it was previously observed for plain nonembedded and therefore freely accessible Cu2−xSe NPs.10,11 Analyzing the subsequently washed particles via TEM (Figure S7) shows that the composite particles have average sizes of 257 ± 28 nm (Hg2+) and 255 ± 22 nm (Ag+). Despite the minimal etching at the ZIF-8 edges, the embedded NPs are still fully encapsulated after treatment with Ag+ or Hg2+ ions for ∼2 h. The elemental mapping in Figure 5 shows that the Cu2−xSe NPs underwent complete ion exchange and were transformed into Ag2Se or HgSe NPs. However, no evidence that the added ions exchanged with the Zn2+ ions in the ZIF-8 framework could be found. These results are further supported by the corresponding XRD patterns (see Figure 3 and Figure S4), showing the clear disappearance of the Cu2−xSe berzelianite diffraction peaks (e.g., at 2Θ values of 26.9° and 44.6°) for both cation exchanges. The appearance of new strong refraction peaks at 2Θ values of 25.5° and 42.0°, which can be assigned to face-centered cubic HgSe (PDF Card No. 00-008-0469) when Hg2+ is added, is proof of the formation of HgSe@ZIF-8 composite particles. While the exchange with Ag+ ions also leads to the disappearance of the berzelianite structure, the new crystal phase formed cannot be identified with absolute certainty due to the low diffraction peak intensities of the resulting silver selenide phase. However, because of previous results for Ag+ cation exchange with Cu2−xSe NPs and the elemental mapping discussed above, the formation of the orthorhombic Ag2Se naumannite phase can be anticipated.10,49

The ZIF-8 windows that connect the larger cavities possess a size of 3.4 Å, and it was shown that even larger molecules can penetrate these through the gate opening effect.52,53 Therefore, we suggest that the cations can diffuse through the ZIF-8 pores to the embedded Cu2−xSe NPs because of their comparable small ionic diameters (0.69 Å for Hg2+ and 1.00 Å for Ag+47), which is similar to the situation discussed for the cation exchange of several MOF metal nodes.38,39,54 This further reveals the accessibility of the embedded NPs, while they are fixed in the MOF framework. We show here an innovative approach to achieving new material combinations for NP@MOF composites. We demonstrate that NP@MOF composites, such as Ag2Se@ZIF-8 and HgSe@ZIF-8, can be synthesized via in situ cation exchange.

4. CONCLUSION In this work, we present the successful integration of two plasmonic semiconductor systems into the nanoporous MOF type ZIF-8. We call them Cu2−xSe@ZIF-8 and ITO@ZIF-8 composite particles. Both the p-type doped Cu2−xSe NPs and the n-type doped ITO NPs preserve their LSPR while being incorporated into the respective ZIF-8 composite structure. The Cu2−xSe@ZIF-8 composite further showed that the LSPR can still be tuned by oxidation, comparable to the case for pristine Cu2−xSe NPs, while the LSPR of ITO@ZIF-8 particles was found to be stable under ambient conditions. Therefore, both systems are potentially interesting for sensory application in which the ZIF network is discriminating on the basis of size the access to the LSPR particles and can be used for optical sensing of redox active substances (Cu2−xSe) or for simply sensing changes in the dielectric surrounding (ITO). Furthermore, we showed that, while being physically protected from the environment and from agglomeration, the NPs are still accessible as can be seen from ion exchange reactions. Via this straightforward strategy, we successfully synthesized HgSe@ZIF-8 and Ag2Se@ZIF-8 composite particles starting from the Cu2−xSe@ZIF-8 composites through an in situ cation exchange that is not destroying the ZIF-8 7516

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials

(10) Wolf, A.; Kodanek, T.; Dorfs, D. Tuning the LSPR in Copper Chalcogenide Nanoparticles by Cation Intercalation, Cation Exchange and Metal Growth. Nanoscale 2015, 7, 19519−19527. (11) Zhang, H.; Xia, Y. Ratiometry, Wavelength, and Intensity: Triple Signal Readout for Colorimetric Sensing of Mercury Ions by Plasmonic Cu2‑xSe Nanoparticles. ACS Sens. 2016, 1, 384−391. (12) Bouet, C.; Laufer, D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.; Patriarche, G.; Dubertret, B. Synthesis of Zinc and Lead Chalcogenide Core and Core/Shell Nanoplatelets Using Sequential Cation Exchange Reactions. Chem. Mater. 2014, 26, 3002−3008. (13) Lesnyak, V.; Brescia, R.; Messina, G. C.; Manna, L. Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9315−9323. (14) Xie, Y.; Bertoni, G.; Riedinger, A.; Sathya, A.; Prato, M.; Marras, S.; Tu, R.; Pellegrino, T.; Manna, L. Nanoscale Transformations in Covellite (CuS) Nanocrystals in the Presence of Divalent Metal Cations in a Mild Reducing Environment. Chem. Mater. 2015, 27, 7531−7537. (15) Miszta, K.; Dorfs, D.; Genovese, A.; Kim, M. R.; Manna, L. Cation Exchange Reactions in Colloidal Branched Nanocrystals. ACS Nano 2011, 5, 7176−7183. (16) Adel, P.; Wolf, A.; Kodanek, T.; Dorfs, D. Segmented CdSe@ CdS/ZnS Nanorods Synthesized via a Partial Ion Exchange Sequence. Chem. Mater. 2014, 26, 3121−3127. (17) Deka, S.; Miszta, K.; Dorfs, D.; Genovese, A.; Bertoni, G.; Manna, L. Octapod-Shaped Colloidal Nanocrystals of Cadmium Chalcogenides via ″One-Pot″ Cation Exchange and Seeded Growth. Nano Lett. 2010, 10, 3770−3776. (18) Akkerman, Q. A.; Genovese, A.; George, C.; Prato, M.; Moreels, I.; Casu, A.; Marras, S.; Curcio, A.; Scarpellini, A.; Pellegrino, T.; Manna, L.; Lesnyak, V. From Binary Cu2S to Ternary Cu-In-S and Quaternary Cu-In-Zn-S Nanocrystals with Tunable Composition via Partial Cation Exchange. ACS Nano 2015, 9, 521−531. (19) Rivest, J. B.; Jain, P. K. Cation Exchange on the Nanoscale: an Emerging Technique for New Material Synthesis, Device Fabrication, and Chemical Sensing. Chem. Soc. Rev. 2013, 42, 89−96. (20) Beberwyck, B. J.; Surendranath, Y.; Alivisatos, A. P. Cation Exchange: A Versatile Tool for Nanomaterials Synthesis. J. Phys. Chem. C 2013, 117, 19759−19770. (21) Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T. Indium Tin Oxide Nanoparticles with Compositionally Tunable Surface Plasmon Resonance Frequencies in the Near-IR Region. J. Am. Chem. Soc. 2009, 131, 17736−17737. (22) Ghosh Chaudhuri, R.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373−2433. (23) Chanana, M.; Liz-Marzan, L. Coating Matters: the Influence of Coating Materials on the Optical Properties of Gold Nanoparticles. Nanophotonics 2012, 1, 199−220. (24) Wolf, A.; Haertling, T.; Hinrichs, D.; Dorfs, D. Synthesis of Plasmonic Cu2‑xSe@ZnS Core@Shell Nanoparticles. ChemPhysChem 2016, 17, 717−723. (25) Eddaoudi, M.; Sava, D. F.; Eubank, J. F.; Adil, K.; Guillerm, V. Zeolite-Like Metal-Organic Frameworks (ZMOFs): Design, Synthesis, and Properties. Chem. Soc. Rev. 2015, 44, 228−249. (26) Gangu, K. K.; Maddila, S.; Mukkamala, S. B.; Jonnalagadda, S. B. A Review on Contemporary Metal-Organic Framework Materials. Inorg. Chim. Acta 2016, 446, 61−74. (27) Hermes, S.; Schroeter, M.; Schmid, R.; Khodeir, L.; Muhler, M.; Tissler, A.; Fischer, R. W.; Fischer, R. A. Metal@MOF: Loading of Highly Porous Coordination Polymers Host Lattices by Metal Organic Chemical Vapor Deposition. Angew. Chem., Int. Ed. 2005, 44, 6237− 6241. (28) Lu, G.; Li, S.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.; Wang, Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.; Chen, X.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang, Y.; Hupp, J. T.; Huo, F. Imparting Functionality to a Metal-Organic Framework Material by Controlled Nanoparticle Encapsulation. Nat. Chem. 2012, 4, 310−316.

framework. Thus, ion exchange shows a possible approach to functionalized ZIF-8 composite particles that might not be accessible via direct synthesis.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03425. Further experimental details; TEM micrographs of Cu2−xSe NPs, ITO NPs, and ZIF-8 particles as well as HgSe@ZIF-8 and Ag2Se@ZIF-8 composite particles; absorbance spectra of Cu2−xSe NPs and in situ cation exchange; and XRD patterns (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the German Research Foundation (DFG) for funding (DFG Research Grants DO 1580/2-1 and DO 1580/3-1). The authors thank the Laboratory of Nano and Quantum Engineering of the Leibniz Universität Hannover and the Volkswagen foundation (lower Saxony/Israel cooperation, Grant ZN2916). T.K. is grateful to the Hannover School for Nanotechnology (HSN) for funding.

■

REFERENCES

(1) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361−366. (2) Dorfs, D.; Härtling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L. Reversible Tunability of the Near-Infrared Valence Band Plasmon Resonance in Cu2‑xSe Nanocrystals. J. Am. Chem. Soc. 2011, 133, 11175−11180. (3) Zhao, Y.; Burda, C. Development of Plasmonic Semiconductor Nanomaterials with Copper Chalcogenides for a Future with Sustainable Energy Materials. Energy Environ. Sci. 2012, 5, 5564−5576. (4) Xie, Y.; Riedinger, A.; Prato, M.; Casu, A.; Genovese, A.; Guardia, P.; Sottini, S.; Sangregorio, C.; Miszta, K.; Ghosh, S.; Pellegrino, T.; Manna, L. Copper Sulfide Nanocrystals with Tunable Composition by Reduction of Covellite Nanocrystals with Cu+ Ions. J. Am. Chem. Soc. 2013, 135, 17630−17637. (5) Kriegel, I.; Rodríguez-Fernández, J.; Wisnet, A.; Zhang, H.; Waurisch, C.; Eychmüller, A.; Dubavik, A.; Govorov, A. O.; Feldmann, J. Shedding Light on Vacancy-Doped Copper Chalcogenides: ShapeControlled Synthesis, Optical Properties, and Modeling of Copper Telluride Nanocrystals with Near-Infrared Plasmon Resonances. ACS Nano 2013, 7, 4367−4377. (6) Comin, A.; Manna, L. New Materials for Tunable Plasmonic Colloidal Nanocrystals. Chem. Soc. Rev. 2014, 43, 3957−3975. (7) van der Stam, W.; Berends, A. C.; de Mello Donega, C. Prospects of Colloidal Copper Chalcogenide Nanocrystals. ChemPhysChem 2016, 17, 559−581. (8) Liu, X.; Swihart, M. T. Heavily-Doped Colloidal Semiconductor and Metal Oxide Nanocrystals: an Emerging New Class of Plasmonic Nanomaterials. Chem. Soc. Rev. 2014, 43, 3908−3920. (9) Hessel, C. M.; Pattani, V. P.; Rasch, M.; Panthani, M. G.; Koo, B.; Tunnell, J. W.; Korgel, B. A. Copper Selenide Nanocrystals for Photothermal Therapy. Nano Lett. 2011, 11, 2560−2566. 7517

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518

Article

Chemistry of Materials (29) Meilikhov, M.; Yusenko, K.; Esken, D.; Turner, S.; Van Tendeloo, G.; Fischer, R. A. Metals@MOFs - Loading MOFs with Metal Nanoparticles for Hybrid Functions. Eur. J. Inorg. Chem. 2010, 2010, 3701−3714. (30) Doherty, C. M.; Buso, D.; Hill, A. J.; Furukawa, S.; Kitagawa, S.; Falcaro, P. Using Functional Nano- and Microparticles for the Preparation of Metal-Organic Framework Composites with Novel Properties. Acc. Chem. Res. 2014, 47, 396−405. (31) Falcaro, P.; Ricco, R.; Yazdi, A.; Imaz, I.; Furukawa, S.; Maspoch, D.; Ameloot, R.; Evans, J. D.; Doonan, C. J. Application of Metal and Metal Oxide Nanoparticles @ MOFs. Coord. Chem. Rev. 2016, 307, 237−254. (32) Hu, P.; Morabito, J. V.; Tsung, C. Core-Shell Catalysts of Metal Nanoparticle Core and Metal-Organic Framework Shell. ACS Catal. 2014, 4, 4409−4419. (33) Roesler, C.; Fischer, R. A. Metal-Organic Frameworks as Hosts for Nanoparticles. CrystEngComm 2015, 17, 199−217. (34) Aguilera-Sigalat, J.; Bradshaw, D. Synthesis and Applications of Metal-Organic Framework-Quantum Dot (QD@MOF) Composites. Coord. Chem. Rev. 2016, 307, 267−291. (35) Zhang, W.; Liu, Y.; Lu, G.; Wang, Y.; Li, S.; Cui, C.; Wu, J.; Xu, Z.; Tian, D.; Huang, W.; DuCheneu, J. S.; Wei, W. D.; Chen, H.; Yang, Y.; Huo, F. Mesoporous Metal-Organic Frameworks with Size-, Shape-, and Space-Distribution-Controlled Pore Structure. Adv. Mater. 2015, 27, 2923−2929. (36) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; UribeRomo, F.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10186−10191. (37) Lalonde, M.; Bury, W.; Karagiaridi, O.; Brown, Z.; Hupp, J. T.; Farha, O. K. Transmetalation: Routes to Metal Exchange within MetalOrganic Frameworks. J. Mater. Chem. A 2013, 1, 5453−5468. (38) Brozek, C. K.; Dinca, M. Cation Exchange at the Secondary Building Units of Metal-Organic Frameworks. Chem. Soc. Rev. 2014, 43, 5456−5467. (39) Deria, P.; Mondloch, J. E.; Karagiaridi, O.; Bury, W.; Hupp, J. T.; Farha, O. K. Beyond Post-Synthesis Modification: Evolution of Metal-Organic Frameworks Via Building Block Replacement. Chem. Soc. Rev. 2014, 43, 5896−5912. (40) Deka, S.; Genovese, A.; Zhang, Y.; Miszta, K.; Bertoni, G.; Krahne, R.; Giannini, C.; Manna, L. Phosphine-Free Synthesis of pType Copper(I) Selenide Nanocrystals in Hot Coordinating Solvents. J. Am. Chem. Soc. 2010, 132, 8912−8914. (41) Cravillon, J.; Schroeder, C. A.; Bux, H.; Rothkirch, A.; Caro, J.; Wiebcke, M. Formate Modulated Solvothermal Synthesis of ZIF-8 Investigated Using Time-Resolved In Situ X-ray Diffraction and Scanning Electron Microscopy. CrystEngComm 2012, 14, 492−498. (42) Tsuruoka, T.; Kawasaki, H.; Nawafune, H.; Akamatsu, K. Controlled Self-Assembly of Metal-Organic Frameworks on Metal Nanoparticles for Efficient Synthesis of Hybrid Nanostructures. ACS Appl. Mater. Interfaces 2011, 3, 3788−3791. (43) Lohe, M. R.; Gedrich, K.; Freudenberg, T.; Kockrick, E.; Dellmann, T.; Kaskel, S. Heating and Separation Using NanomagnetFunctionalized Metal-Organic Frameworks. Chem. Commun. (Cambridge, U. K.) 2011, 47, 3075−3077. (44) Saldanha, P. L.; Brescia, R.; Prato, M.; Li, H.; Povia, M.; Manna, L.; Lesnyak, V. Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide, and Telluride-Sulfide Nanoparticles. Chem. Mater. 2014, 26, 1442−1449. (45) Demessence, A.; Boissiere, C.; Grosso, D.; Horcajada, P.; Serre, C.; Ferey, G.; Soler-Illia, G.; Sanchez, C. Adsorption Properties in High Optical Quality NanoZIF-8 Thin Films with Tunable Thickness. J. Mater. Chem. 2010, 20, 7676−7681. (46) Cookney, J.; Ogieglo, W.; Hrabanek, P.; Vankelecom, I.; Fila, V.; Benes, N. E. Dynamic Response of Ultrathin Highly Dense ZIF-8 Nanofilms. Chem. Commun. 2014, 50, 11698−11700. (47) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, Internet Version 2005; CRC Press: Boca Raton, FL, 2005.

(48) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (49) Miszta, K.; Gariano, G.; Brescia, R.; Marras, S.; De Donato, F.; Ghosh, S.; De Trizio, L.; Manna, L. Selective Cation Exchange in the Core Region of Cu2‑xSe/Cu2‑xS Core/Shell Nanocrystals. J. Am. Chem. Soc. 2015, 137, 12195−12198. (50) Li, H.; Zanella, M.; Genovese, A.; Povia, M.; Falqui, A.; Giannini, C.; Manna, L. Sequential Cation Exchange in Nanocrystals: Preservation of Crystal Phase and Formation of Metastable Phases. Nano Lett. 2011, 11, 4964−4970. (51) Luther, J. M.; Zheng, H.; Sadtler, B.; Alivisatos, A. P. Synthesis of PbS Nanorods and Other Ionic Nanocrystals of Complex Morphology by Sequential Cation Exchange Reactions. J. Am. Chem. Soc. 2009, 131, 16851−16857. (52) Diestel, L.; Bux, H.; Wachsmuth, D.; Caro, J. Pervaporation Studies of n-Hexane, Benzene, Mesitylene and their Mixtures on Zeolitic Imidazolate Framework-8 Membranes. Microporous Mesoporous Mater. 2012, 164, 288−293. (53) Kolokolov, D. I.; Diestel, L.; Caro, J.; Freude, D.; Stepanov, A. G. Rotational and Translational Motion of Benzene in ZIF-8 Studied by 2H NMR: Estimation of Microscopic Self-Diffusivity and Its Comparison with Macroscopic Measurements. J. Phys. Chem. C 2014, 118, 12873−12879. (54) Fei, H.; Cahill, J. F.; Prather, K. A.; Cohen, S. M. Tandem Postsynthetic Metal Ion and Ligand Exchange in Zeolitic Imidazolate Frameworks. Inorg. Chem. 2013, 52, 4011−4016.

7518

DOI: 10.1021/acs.chemmater.6b03425 Chem. Mater. 2016, 28, 7511−7518